Dynamin is a large, extended protein consisting of five domains. We have recently determined the 2.0 Angstrom resolution crystal structure of a human dynamin 1-derived minimal GTPase-GED (for "GTPase effector domain") fusion protein. This fusion protein dimerizes in the presence of the transition state mimic GDP-aluminum fluoride. The structure reveals dynamin's catalytic machinery and explains how assembly-stimulated GTP hydrolysis is achieved through G domain dimerization. Particularly intriguing was the identification of a sodium ion in the active site, suggesting that dynamin uses a cation to compensate for the developing negative in the transition state and providing a rationale for the inability to previously implicate a more-usual arginine finger. The structure of this fusion domain allows us to provide a model for the role of dimerization during dynamin-catalyzed membrane fission.